From Fertilization to Fetus
- We’ve already learned how
gametes are produced
(gametogenesis) through
- What process produces
every other cell in the
human body?
• When a baby girl is born, she contains
millions on follicles in her ovary
• After puberty, every cycle (28 days) some
of these follicles begin to develop (FSH)
but only one “wins”
• This “winning” follicle is the one that will
ovulate an egg (ruptures thanks to LH)
• The ruptured follicle then turns into a
corpus luteum (yellow body) that pumps
out progesterone to get ready for
pregnancy (creates uterine wall)
• The ruptured egg, meanwhile, is hanging
around in the fallopian tubes waiting to
get fertilized
Journey of the Sperm
• The sperm (in human reproduction) gets deposited in the vagina
• The female reproductive system is lined (thanks to progesterone) with a
viscous alkaline mucous that the sperm swim through
• The sperm travels through the cervix (a huge selective barrier), through the
uterus, and up the fallopian tubes where the egg is waiting
Mammalian Fertilization
1. Sperm binds to receptors in zona pellucida (extracellular matrix of egg)
2. Acrosomal reaction: sperm releases hydrolytic enzymes to digest z.p.
Depolarization of membrane: prevent other sperm from binding = fast block to
3. Sperm + Egg Fuse
4. Cortical reaction: sperm + egg fusion triggers release of Ca2+
 cortical granules fuse with z.p.  z.p. hardens to form fertilization
envelope = slow block to polyspermy
5. Ca2+ release also triggers activation of the egg
Blocking Polyspermy
• The sperm and eggs plasma membrane’s fuse and this triggers a fast
depolarizing reaction that blocks other sperm trying to enter the cell
• If not, you can get fraternal twins!
• https://www.youtube.com/watch?feature=player_embedded&v=T6BtSMerB
• https://www.youtube.com/watch?v=7G2rL5Cutd4
Fertilization  Zygote
• the sperm releases it’s
pronucleus into the egg’s
• The sperm and egg’s pronuclei
(1n) fuse to create a diploid
nucleus (2n)
• We now have a zygote! This is
the very first cell that was every
Zygote  Morula
• The zygote undergoes several
mitotic divisions (cleavage) over
a 3-4 day period
• 2-cell  4-cell  8-cell  16 cell
• Finally it becomes a (16-celled)
morula. It is still a solid ball of
cells with defining shape
Morula  Blastocyst
• Slowing, the morula cavitates and the inner cells begin to migrate to
one small section of the embroyo forming the inner cell mass (ICM)
• The outer, trophoblast cells will become the placenta
• The ICM cells will become the embryo
Implantation of the Blastocyst
• As the fertilized egg developed from zygote to blastocyst, it was being moved
down the fallopian tubes via smooth muscle contractions to the uterus
• In apes (not like this in most species), the blastocyst actually digs through the
uterine wall causing some women to experience implantation bleeding
Blastocyst  Gastrula
• Gastrulation – The outer-wall of the blastocyst invaginates on itself
(folds in) and three distinct germ layers form
• A germ layer is a primary layer of cells that form during
• These layers become specific structures
• Ectoderm
• Mesoderm
• Endoderm
Gastrulation: rearrange cells to form 3-layered embryo
w/primitive gut
Three Embryonic Germ Layers*
• Skin, nails, teeth
• Lens of eye
• Nervous system
(brain, spinal cord)
• Skeletal, muscular
• Epithelial linings of
digestive, respiratory,
excretory tracts
• Notochord
• Excretory, circulatory • Liver, pancreas
• Reproductive system
• Blood, bone, muscle
* For AP Test, you should know at least 2 derivatives of each germ layer.
Gastrula  Embryo
• After gastrulation, organogenesis and
neurulation begins
• Embryo development is a major piece of
evidence for evolution
Organogenesis: development of 3 germ layers into
• Notochord – stiff dorsal skeletal
rod, forms from mesoderm
• Neural plate  neural tube
brain and spinal cord
• Neurulation – forms hollow
dorsal nerve chord
• Somites – blocks of mesoderm
arranged along notochord; sign
of segmentation
Protosomes vs. Deuterostomes
• In protostomes the mouth opening
is the first to be formed later
followed by the anus. In
deuterostomes the anus forms first
followed by the mouth. This all
occurs in the gastrulation stage.
Patterns of development
• Cytoplasmic determinants: chemical signals such as mRNAs and
transcription factors, influence pattern of cleavage
• Induction: interaction among cells that influences their fate, cause
changes in gene expression
• Totipotent cells: capable of developing into all the different cell types
all cells of mammalian embryos are totipotent until the 16-cell stage